The present disclosure relates to nickel-based alloys and articles based thereupon.
Gas turbine engines operate in extreme environments, exposing engine components, especially those in the turbine section, to high operating temperatures and stresses. Power turbine buckets (or blades) in particular, which may be up to, or over, about 36 inches long and weight up to, or over, about 40 pounds, require a balance of properties including, but not limited to, casting cracking resistance, tensile strength, ductility, creep resistance, oxidation resistance, hot corrosion resistance, low freckle susceptibility, sufficiently low density, reasonable cost, and a moderately large heat treatment window.
Superalloys have been used in these demanding applications because of their ability to maintain reasonably high strengths up to ˜75% of their respective melting temperatures, in addition to having excellent environmental resistance. Nickel-based superalloys, in particular, have been used extensively throughout gas turbine engines, e.g., in turbine blade, nozzle, and shroud applications. However, conventional nickel-based superalloys used in latter stage bucket applications can be difficult to cast, resulting in low yield. The steady increase in gas turbine firing temperature requirements has historically relied upon improved mechanical and environmental material performance in these applications.
Directional solidification has been successfully employed to optimize creep and rupture behavior in nickel-based superalloy bucket applications. Preferentially orienting grains in the direction of the principal stress axis, which generally coincides with the longitudinal direction, provides a columnar grain structure, eliminating grain boundaries transverse to the growth direction. Such an orientation also provides a favorable modulus of elasticity in the longitudinal direction, beneficial to the fatigue performance of the part.
When compared with conventionally cast alloy articles, the application of the directional solidification process produces articles having significant improvements in strength, ductility, and resistance to thermal fatigue. However, reduced strength and ductility properties may still be seen in the transverse direction in such articles due to the presence of columnar grain boundaries. In efforts to improve the transverse grain boundary strength of such articles, additional alloying elements, e.g., hafnium, carbon, boron and zirconium, have been utilized. However, the addition of these elements, as well as others, can result in a depression in other desired properties, e.g., melting point, and so a compromise in the balance of properties has heretofore been required.
Thus, there remains a need for nickel based alloys that exhibit more, or substantially all, of the desirable properties for use in gas turbine engines, e.g., resistance to corrosion, oxidation and creep, as well as high temperature strength. It would further be desired if any alloys so provided would either comprise elements not substantially detrimental to the desired properties, or be processed in such a way that any detriment to the desired properties is minimized, or eliminated.